Influence of soil nitrogen availability on plant allocation patterns: Allelochemical response varies with developmental stage and among plant parts

Background/Question/Methods

Soil nitrogen availability plays a key role in plant resource allocation to growth, reproduction, and defense. Human nitrogen inputs in terrestrial ecosystems have dramatically increased in recent years and these inputs may alter plant allocation patterns, plant defense, and plant-herbivore interactions. Although a number of studies have investigated the influence of nutrient availability on plant allocation patterns, few have addressed the effects of nitrogen enrichment alone and within a range of predicted increase. We conducted two greenhouse experiments to examine the influence of simulated nitrogen (N) enrichment on plant growth, reproduction, allelochemistry, and allocation trade-offs. Specifically, we investigated the effects of N availability on an invasive plant, Dalmatian toadflax (Linaria dalmatica, Plantaginaceae). This plant produces iridoid glycosides, which are a group of allelochemicals that can act as defenses against generalist herbivores and pathogens as well as attractants and feeding stimulants for specialist herbivores. 

Results/Conclusions

As expected, N enrichment increased plant growth and investment in reproduction. However, the effects of N availability on allelochemistry were more complex and depended on developmental stage and intra-plant allocation patterns. During early plant development (pre-flowering), iridoid glycoside concentrations in leaves and stems decreased with increased N. However, for flowering plants, N enrichment resulted in no change in iridoid glycoside concentrations of leaves and stems, whereas concentrations decreased in flowers and increased in roots. Graphical vector analyses revealed that iridoid concentrations most likely varied due to changes in biomass as well as changes in allelochemical synthesis and/or allocation. We found no indication of trade-offs between plant growth and defense, but some evidence of trade-offs between investment in reproduction and defense. Moreover, our results suggest that costs of defense can be mediated by soil nitrogen availability. This study highlights the importance of examining intra-plant variation and developmental shifts in allelochemistry in order to characterize and predict plant response to nutrient availability and environmental change.